Boiler for heating non-boiling heat transfer liquids

ABSTRACT

This invention relates to an improvement in a vertical boiler for heating non-boiling heat-transfer liquids having two tightly wound axially symmetrically coiled pipe walls concentrically positioned at a distance with respect to each other inside an axially symmetrical housing, the external wall extending to the boiler bottom and radially delimiting the outside of a riser for flue gas, said riser surrounding the internal wall, aperture means in the upper end of said housing for the introduction of heating means, and a coiled pipe spiral constituting the boiler bottom, the improvement comprising an internal pipe wall which extends to the boiler bottom, at least said internal wall with the lowermost pipe winding being supported by the boiler bottom, and gaps between the lower coils of said internal pipe wall forming a connection between the space enclosed by said internal wall and said riser.

United States Patent [11] 3,822,675

Rychen July 9, 1974 [541 BOILER IEAEN$NBOILING FOREIGN PATENTS ORAPPLICATIONS HEAT TRAN FER Q 1,997,746[U] 3/1968 Germany 122/250 [75]Inventor: Niklaus Rychen, Kanton Zurich,

Switzerland Primary Examiner-Kenneth W. Sprague [73] AssigneezNorda|pina Anstalt, Attorney, Agent, or Firm-James E. Bryan, Esq.

Vaduz/Furstentum, Liechtenstein 57 ABS CT [22] Filed: Mar. 26, 1973 Th1s1nvent1on relates to an Improvement In a vertical PP 344,564 boiler forheating non-boiling heat-transfer liquids having two tightly woundaxially symmetrically coiled [30] Foreign Application Priority Data plpewalls concentncally positioned at a distance with respect to each other1ns1de an axially symmetrlcal Mar. 28, Germany housing, the externalwall extending to the boiler bottom and radially delimiting the outsideof a riser for g 122/250 flue gas, said riser surrounding the internalwall, aperd 249 250 R ture means in the upper end of said housing forthe in- 1 0 arc 1 6 troduction of heating means, and a coiled pipespiral constituting the boiler bottom, the improvement comprising aninternal pipe wall which extends to the [56] References cued boilerbottom, at least said internal wall with the low- UNlTED STATES PATENTSermost pipe winding being supported by the boiler 1,700,961 2/1929Vaughn 122/250 bottom, and gaps between the lower coils of said inter-2,l60,644 5/1939 Clarkson..... 122/250 nal pipe wall forming aconnection between the space 2,20] ,620 5/l940 LaMont t enclosed aidinternal wall and aid risen 2,645,210 7/1953 Harris et al. 122/2503,05l,l46 8/l96 2 Clarkson et al 122 249 17 Claims, 8 Drawing FiguresPATENTED JUL 1 74 SHEET 1 [1F 2 Fig.3

Fig.4

PATENTEDJUL 91w 3,822,675

SHEU 2 BF 2 hnulli "Ill.

BOILER FOR HEATING NON-BOILING HEAT TRANSFER LIQUIDS The presentinvention relates to a vertical boiler for heating non-boiling heattransfer liquids, particularly oils, with two tightly wound axially, orrotationally, symmetrical coiled or serpentine pipe side walls beingconcentrically disposed, one within the other, and at a distance withrespect to each other, inside an axially symmetrical boiler housing. Theinternal wall radially delimits the central furnace, and the externalwall extends to the boiler bottom and radially delimits toward theoutside a riser enclosing the internal wall and therewith also theportion of the boiler through which flue gas flows. A feed line extendsinto the boiler through the upper end wall for an air-fuel mixture orfor hot flue gas, and a coiled or serpentine pipe spiral constitutes theboiler bottom, which preferably drops downwardly toward the center.

Boilers of this particular type are known from Ger. Gebrauchsmuster No.1,997,746.

While it is preferred, according to the present invention, that thecoiled pipe walls as well as the boiler housing are constructed so as tobe cylindrical, the configuration of the coiled pipe walls and that ofthe boiler housing in the prior art construction is conical. In theprior art construction, the internal coiled pipe wall terminates at aconsiderable distance from the boiler bottom, so that below this wallthe initially downwardly flowing flue gases with turn around and beadapted to flow upwardly, in the space or chamber between the internaland the external walls in which a further coiled pipe arrangement isprovided for, until they reach the stack of the boiler. Both in thepresent invention and also in the prior art construction, a burner, andpreferably an oil burner, is disposed on top of the preferably removablelid or cover of the boiler.

In comparison with this prior art construction, the present inventionprovides for a boiler of the type outlined hereinabove in which thethermal load or charge of the pipes in the various coiled pipe walls canbe determined very precisely, whereby the danger of local overheating ofthe coiled pipes, and thus of a harmful action of the heat carrierflowing into the heating tube coils is reduced to a minimum.

Further, in the boiler proposed by the present invention, the coiledpipe construction constitutes the supporting structural element of theboiler itself so that the boiler housing now serves merely for purposesof insulation. The combination of these advantageous features isrendered possible, according to the present invention, by virtue of thefacts that the internal wall also extends to the boiler bottom; that atleast the internal wall with the lowermost tube or pipe coil issupported by the boiler bottom, and that the connection between thefumace'and the riser isestablished by the gaps in the lower portion ofthe internal wall being wound with a distance. Due to the fact that thedeflection or return of the flue gases from the furnace into the riseris accomplished through gaps in the internal wall, and that these gapsalso still are positioned in the lower area of the internal wall,practically the entire heat radiation of the flame burning in thefurnace is absorbed from the internal wall, which latter may becorrespondingly dimensioned. The external wall, or even severaloutwardly situated coiled pipe walls then must be dimensioned only insuch a fashion that they will faultlessly withstand the heat absorptionby convection. As a consequence of the construction as describedhereinabove it becomes possible, in turn, to cause the internal wall,which is as a rule the heaviest and strongest, to be seated directlyupon the boiler bottom, so that the boiler bottom may beutilized as asupporting element for the internal wall, and preferably also theexternal wall.

Due to the subdivision of the coiled pipes leading the heat carrier intotwo strictly different or separate groups, one of which is heated byconvection and radiation from the furnace, whereas the other one isheated merely by convection by the flue or boiler gases that have leftthe furnace, the thermal load of the coiled pipes and the flow velocityof the heat carrier therein may be calculated very accurately so thatmaximal heating is achieved without the danger of overheating.

The gaps between the various serpentine pipe coils or turns in the lowerregion of the internal wall extend advantageously approximately over thelower 20 percent of the total height of the internal wall.

Another advantage of the construction proposed by the present inventionis that, by reason of the precise dimensioning of the gaps, the pressuredrop inside the boiler, and therewith the weight rate of flow of theflue gas may be accurately dimensioned.

Due to the fact that the invention as is generally known per se delimitsthe space through which flue gas flows toward the outside by means ofthe external tightly-wound heating coil pipe wall, no uncooled surfacespractically will be present in the entire furnace, which increases thesafety considerably. Since in the external wall standing upright on theboiler bottom with a helically-extending lowermost turn or coil, such asis preferred, a gap remains free between the bottom and the lowermostcoil or turn, it is preferred in this construction that the externalwall encloses or sur rounds the boiler bottom from the outside so thatthe lowermost coil does not extend over but next to the boiler bottom sothat such a gap is eliminated.

In this manner acts also the boiler bottom construction used in thepresent invention which prevents the heat storage in the otherwisecustomary bricked boiler bottoms. This is of particular significance inboilers serving for heating heat transfer liquids which do not boil,since heat carriers of this type are sensitive to overheating as a rule.When the circulating pump in a heating system including such a heatcarrier breaks down, it is quite possible that in boilers which are notconstructed as proposed by the present invention, local overheating, andtherewith possibly even the destruction of the entire boiler system, maybe produced much more easily.

In order to assure a flawless support of the internal wall by the boilerbottom, the lowermost coil or turn of the internal wall extendspreferably in a helical manner so that it will rest on the one handalong a spiral line against the boiler bottom, and on the other handalso against the next higher and helically-extending coil or turn of thetube or pipe constituting the wall. In this manner, the lowermost coilcarries the entire wall. In principle, it may extend in a spiral manneralso toward the outside, but it is preferred that the lowermost coil orturn of the internal wall reduces its radius toward the end thereofbeing farther removed from the wall.

The gaps between the lower coils are advantageously retained true tosize or dimension in each case only with spacer means being welded toone of the two tube or pipe coils delimiting the gap. This assures amaximal yielding of the entire construction with regard to ther malstresses, which in turn increases the service life and the carryingcapacity of the boiler. The spacer means advantageously haveapproximately the shape of short rods which thicken again toward the twoends thereof.

The pipes constituting the walls preferably have a circularcross-section in the usual manner.

The lowermost coil of the internal wall may terminate in the boilerbottom either centrally downwardly, or on the outside. The latterarrangement is preferred since as a result an excessive exposure of thelast end of the lower coil with respect to the flame is effectivelyprevented.

For purposes of improving the thermal efficiency, a third coiled pipewall may be disposed between the two walls in the riser, which ispreferably wound or coiled at a distance. This is known per se. But evenfurther coiled pipe walls are possible. Moreover, additional fallers andrisers may be disposed between the external wall and the internal wall.Preferred, however, is the construction as outlined at the beginning ofthis paragraph.

The coiled pipe walls are preferably radially held at a distance withrespect to each other by means of axial rods, as is also known per se.But in contrast to the state ofthe art, the axial rods according to thepresent invention are preferably not welded to the coiled pipe walls,but are simply inserted between the latter. As a result, the axial rodsmay be exchanged with a minimum amount of expenditure. Anothersignificant advantage of this construction is that no weakening of thecoiled pipe walls is caused by welded joints with these spacer rods.This rod arrangement too is rendered possible only as a result of thestrict separation between the radiating portion and the convectionportion.

The welded joints between the various pipes constituting the coiled pipewalls are preferably so provided for that the welding seam projectsneither outwardly nor inwardly. This facilitates not only the tightwinding or coiling, but in addition thereto prevents inaccuracies in theflow of the heat carrier, and the formation of dead spaces which may, inturn, lead to harmful actions of the heat carrier.

The internal pipe wall is preferably connected at the top to the returnconnection of the boiler and at the bottom to the external wall whichadjoins at the top the boiler flow or lead connection.

The arrangementjust described above has the advantage that the mostthermally stressed internal wall is passed through by the relativelycool heat carrier which has already given off its heat.

The third wall is preferably disposed parallel to the external wall.

The inventive provision of the boiler with a selfsupporting heating coilsystem renders it possible as is indeed preferred that the uppermostcoil of the internal wall carries the cover of the boiler. For this purpose, this coil extends likewise advantageously in a spiral fashion. Incontrast to the lowermost coil, the spiral extends in this case,however, advantageously outside of the second highesthelically-extending coil.

In order to assure a perfect closure of the cover during the thermalexpansions of the heating coil walls and during the correspondinglifting and lowering of this cover, the latter is preferably equippedwith a circumferential collar which is immersed into a groove beingupwardly open and filled with a pourable or plastic sealing material,and which groove is positioned in turn at a sheet metal sleevesurrounding or enclosing the external pipe wall. The groove may befilled, for example, with sand or slag.

The introduction of the lowermost coil of the internal wall into theboiler bottom is effected advantageously in a direction such that theend of the spiral channel in the bottom is rinsed by the liquid. Thisconstruction is to be preferred whenever the boiler bottom is formed bytwo embossed sheets or plates between which extends a spirally providedweb which constitutes the spiral channel for the heat carrier. Since itis not possible to connect, without a considerable technicalexpenditure, the lowermost coil of the internal wall in aflowtechnically perfect manner'to such a spiral channel, the formationof dead spaces can be avoided on the basis of the inventive teaching.For this purpose it is sufficient that the end of the pipe forming thelowermost coil is allowed to further project a short distance at leaston one side into the boiler bottom, and that this end has a curvaturesuch that the flow is initially directed into the corner otherwiseforming a dead space. It is equally possible to simply weld on acorresponding reversing sheet or plate.

The lowermost coil of the external wall preferably extends in a helicalmanner and is supported, by means of supporting blocks being weldedthereto, against the boiler bottom, or a flange surrounding the same.These supporting blocks are preferably welded to the pipe forming thecoil with considerably longer welding seams than to the boiler bottom orthe flange. in case the welding seam breaks here due to thermalexpansions, it will not break at the connecting point to the pipethereby endangering the boiler.

The lowermost coil of the internal wall, which carries the internalwall, is advantageously supported from the inside by means of aspirally-extending rod welded to the boiler bottom. Instead of a rodextending all around, shorter rod pieces also may be welded to theboiler bottom and will secure the lowermost coil by contact from theinside. in principle, such supporting rod pieces may be applied also onthe outside.

In the boiler bottom used according to the present invention it isessential that the flow perfectly follows the theoretical or desiredspiral path. This was not always assured in the heretofore knownconstructions since the spirally-extending web between the two sheets orplates was welded only to one of the two sheets.

This difficulty is circumvented, according to the present invention, inthat the vertical sheet band spiral extending between the two sheetsconstituting the boiler bottom is butt-welded to one sheet, preferablythe sheet facing the furnace, whereas the other edge thereof extendsthrough a spiral slot of the other sheet and is welded with the latterfrom the outside.

The present invention will now be described hereinafter in furtherdetail on the basis of and with reference to the schematic drawingswhich show the subject matter of the invention in a preferred embodimentthereof, and wherein FIG. I is an axial view through a boiler accordingto the present invention,

FIG. 2 illustrates the boiler flow connection to the heating coil walls;

FIG. 3 illustrates the arrangement of a spacer means or member in thegap between two heating coils of the internal wall at an enlarged scaleas compared to FIG. 1;

FIG. 4 is a perspective view of a spacer means or member at an enlargedscale as compared to FIG. 3;

FIG. 5 illustrates the cover seal of the boiler at an enlarged scale ascompared to FIG. 1;

FIG. 6 illustrates the type of welding of a spiral web band into the twosheets or plates constituting the boiler bottom at an enlarged scale ascompared to FIG. 1,

FIG. 7 illustrates the welding of a supporting rod on to a supportingcoil of a boiler wall, and

FIG. 8 is a top plan view of the two lowermost coils of the internalheating coil wall.

The boiler proposed by the present invention and illustrated in theaccompanying drawings is equipped with a supporting bottom ring 1 fromsteel from which upwardly extend, for example, four vertical web sheetsor plates 2. Welded to the upper edges of these web plates as isapparent from the drawings is a ring 3 having an angle section and whosevertical web carries the boiler bottom 4. The boiler bottom 4 consistsof two embossed sheets or plates 5 and 6 which are welded with theperipheral edges or rims thereof to the vertical web of the angle ironsection 3. Disposed between the embossed plates 5 and 6 extending at adistance with respect to each other is a sheet band spiral 7 which formsin the boiler bottom, together with the two plates 5 and 6, a heatingcoil spiral passed through by the heat carrier. The sheet or plate bandspiral 7 is welded with the upper rim thereof to the plate 5, as is moreclearly apparent from FIG. 6, whereas the lower rim of this plate bandspiral 7 extends through a spiral slot in the lower plate 6 and istightly welded from the outside with the two edges of this slot, as isequally apparent from FIG. 6. As a result of this construction, thecoiled pipe spiral formed by the boiler bottom is absolutely tight, andshort-circuiting is not possible.

Seated upon the edge or rim of the boiler bottom is the internal coiledpipe wall 8. It is evident from the drawings that between the lowermostfour, purely spirally extending coils of the coiled pipe wall 8 bywinding the latter with interstices, gaps 9 have remained free throughwhich the flue gases may enter from the furnace 10, being enclosed orsurrounded by the coiled pipe wall 8, radially outwardly into the riser11. In the zone or area above the lower four spiral windings of the wall8, the latter is not so coiled or wound so that a radial issuance ordischarge of flue gases and flame radiation toward the outside is notpossible in this area.

Adjoining the lowermost helically-extending winding 12 of the wall 8that has again been shown completely in a top plan view in FIG. 8, is afurther spirallyextending winding 13 which, beginning from theconnection thereof to the pipe 12, as indicated in FIG. 8 by a radiusline, will constantly decrease its radius in a manner such that thewinding 13 will, on the one hand, rest on the entire length thereofagainst the plate 5 of the boiler bottom and, on the other hand, carrythe superimposed helically-extending pipe winding 12 also on the entirelength thereof, along a line of contact. The

pipe winding 13 is welded at 14 to a correspondingly provided intakeconnection 15 in the boiler bottom. The intake connection 15 passesthrough the upper boiler bottom plate 5 with an oval opening. Its wallportion being disposed on top in FIG. 8 and behind the drawing plane inFIG. 1 projects as is visible in FIG. 1 into the outermost heating coilwinding of the boiler bottom so that a downwardly-directed heat carrierflow in the coiled pipe wall 8 will initially be slightly reversed ordeflected in the opposite direction so that it rinses the anglesexisting there at the beginning of the heating coil spiral in the boilerbottom.

In order to hold the lowermost winding 13 securely on the boiler bottom,to which it is incidentally not further welded, short round steel rodpieces 16 are so welded to the boiler bottom at a distance with respectto each other that they will lie precisely flush along a line of contactagainst the coiled pipe winding 13. As a result, it is secured against alateral displacement due to the influence of thermal expansions. Itcarries the entire weight of the internal coiled pipe wall 8. In orderthat the latter not be compressed due to its own weight and close thegaps 9, spacer means or members 17 are inserted into the gaps 9 whosepreferred specific shape is apparent from FIGS. 3 and 4. The spacermembers 17 are welded in each case only to one of the two windings 18and 19 being held at a distance by them, or are simply inserted. Byvirtue of the concave configuration or curvature of their centralportion they not only maintain the correct distance or spacing; theyconstitute at the same time a radial guide for the pipes being separatetherefrom. When the spacer members are provided with nose portionsextending to the next pipe wall, these nose portions may replace theaformentioned spacer members at least partly.

At the upper end the return connection 20 adjoins the internal pipe wall8.

Provided adjoining the uppermost helicallyextending winding of theinternal pipe wall 8 is a further spirally-extending winding 21 whichterminates in the connection 20. The uppermost winding 21 extends in amanner such that, on the one hand, it rests on its entire length alongone line against the upper winding of wall 8 extending along a helicalline and, on the other hand, along a spiral line extending in a plane atthe boiler cover 22. In this manner, the uppermost winding of theinnermost wall 8 carries the cover 22 of the boiler, which is adapted tobe lifted and lowered with the thermal expansions of this winding. Inthe center, the cover 22 has an aperture 23 for the insertion of,

' for example, an oil burner. For purposes of sealing with respect tothe boiler housing 24 which essentially consists of two coaxial sheet orplate jackets 25 and 26, having the extension apparent from the drawingsand whose interstices are filled with a suitable insulating material,such'as, for example, a glass wool, the cover 22 is surrounded by adownwardly open U-shaped, or T-shaped, or angle section rod 27 bent toform a ring. The outer leg of the ring 27 thus formed is immersed intothe interior of an upwardly open U-shaped section ring 29 which issecured in a suitable manner to the top of the boiler housing 24. Thearrangement is so made that within the thermal expansions to beexpected, the outer leg of the U-shaped section ring 27 will not reachthe bottom of the outer U-shaped section ring 29. For purposes of aperfect sealing, sand 31, for example, is placed in the outer U-shapedsection ring 29.

It is apparent from the foregoing description that the internal coiledpipe wall 8 constitutes a unit in a selfsupporting construction with theinternal boiler bottom 4 which rests by way of the web plates 3 on oragainst the supporting ring 1. Nevertheless, a maximum movability of thewindings of the internal wall 8 with re spect to each other is assured.

From below, there adjoins the center of the heating coil spiral formedby the boiler bottom an approxi mately radially extending pipe line 30for the heat carrier liquid which at the end is connected to theexternal coiled pipe wall 32. This external pipe wall is also tightlycoiled or wound, like the upper part of the internal wall 8, so that theflue gases flowing upwardly between the internal wall and the externalwall in the rise will not come into contact with the internal plate 26of the boiler housing.

From the description contained hereinabove it is apparent that in theinternal boiler area which comes into contact with the flue gases, thereare no parts not cooled by heat carrier liquid which could constituteheat containers or reservoirs of such size or extent that the heatstored therein, in case of a breakdown of the circulation of the heatcarrier, could pass over onto the heat carrier now standing still andadversely affect the latter.

In the external coiled pipe wall 32, the heat carrier liquid, whichflows helically downwardly in the internal wall, flows helicallyupwardly. From the uppermost winding of the wall 32, a short pipeconnection 33 projects upwardly and in turn passes over into thedischarge connection 34 of the boiler. The discharge connectionpreferably extends parallel to the return connection at the same heightin FIG. 1, slightly ahead of the drawing plane.

In the embodiment shown, which is also preferred, there are disposedbetween the internal coiled pipe wall 8 and the external coiled pipewall 32 at least one and preferably a further central coiled pipe wall35 being passed through parallel to the external coiled pipe wall 32,and whose various windings extend on the entire height of this wall at adistance with respect to each other so that these pipes arecircumcirculated particularly well by the flue gas stream. It is alsopossible to provide spacer means or members of the type of the spacermembers 17 between the pipes of this wall. These spacer members arelikewise advantageously firmly welded to one of the two pipe windingsbeing maintained at a distance by them. At the lower end, the lowermostwinding of the intermediate wall 35 also connects to the line 30. Theupper end of the wall 35 is also connected to the discharge connection34, as is apparent from the drawing.

While in smaller boilers the internal wall 8 and the external walls arepreferably constructed in a singlethreaded manner, the external wall 32,the central wall 35 and also the internal wall in larger boilers arepreferably constructed in multiplethread.

Also the external wall 32 and the central wall 35 rest in aself-supporting manner on the horizontal flange of the angle sectionring 3, as is evident particularly from FIG. I. Since also the lowermostwinding of these two walls. in the embodiment shown rises upwardly in ahelical fashion without a spiral supporting winding, round steel pieces40, as shown in FIG. 7, are welded at a distance to the respectivelylowermost winding 41. The round steel pieces 40 are in this casepreferably welded with two relatively long welding seams to the pipewinding 41, and only with a small short welding seam to the horizontalleg of the angle section ring 3. If damage arises here due to thermalexpansion or other forces, it may result merely in a break of thewelding seam between the round steel piece and the leg 3, but cannotproduce a break of the welding seams be tween the round steel piece 40and the pipe 41.

The round steel pieces 40 are placed so closely from within or fromwithout or even from both sides against the corresponding pipe winding41, in each case prior to the welding operation, that it will beperfectly carried. This means that already prior to welding, the roundsteel piece 40 should in each case rest along a line of contact againstthe horizontal leg of the angle section 3 and also against the pipesection 41 along a line of contact.

In order to fix or secure the three coiled pipe walls also in the radialdirection, rods 45 from a difflculty oxidizable grade of steel areinserted between the internal pipe wall and the central pipe wallradially and at a distance with respect to each other. These rods arepreferably connected at the upper ends thereof as is appar ent from FIG.1 with further spacer rods 46 by means of welding with the aid of ashort web piece, and these spacer members 46 maintain the central wall35 and the external wall 32 at a distance. In the embodiment shown, theexternal rods 46 have circular indentations with which they keep thevarious pipes of the central wall 35 at a distance from each other.There is no need for using both of the possiblities shown formaintaining a spacing of the various windings of the central wall 35.Either spacer members or the notches in the external spacer rods 46 canbe used for this purpose. If spacer members are present, preferablyround steel rods or other rods are inserted as spacer rods, having asection which will not tilt even during tension between the walls, i.e.,for example a square profile, or an equilateral triangular profile, or aflat profile whose largest extension extends in the circumferentialdirection.

When the spacer rods 45 and 46 must be exchanged, it is sufficient toseparate the bridge connecting them at the upper end thereof. At thattime, the rods 45 may be readily pulled out upwardly. The rods 46 have aflat iron profile and at the narrow edge or rim thereof the circularlybent indentations must be so turned prior to being pulled out that thecircular-shaped notches will be freed from the pipes of the wall 35.Then they too may be pulled out upwardly and replaced by other rods. Theheat capacity of these spacer rods 45 and 46 is so low that even in caseof a breakdown of the circulating pump a harmful influence or action ofthe heat carrier inside the pipe coils by the heat being stored in therods will be avoided with certainty.

In the embodiment shown, the hot flue gas generated in the furnace orintroduced into the latter from above flows initially downwardly in thefurnace 10 where it heats the internal wall 8 and the boiler bottom byconvection and radiation. Thereafter, the flue gas flows through thegaps 9 radially outwardly into the area between the wall 8 and the wall32, where equally circumcirculating the central wall 35 it flowsupwardly, so as to leave the boiler through the stack 50.

In the embodiment shown, dead spaces at the beginning and end of thebottom spiral are avoided by corresponding deflecting arrangements;instead it also would be possible to provide there apertures in the wallto the next-following or preceding winding of the spiral. Theseapertures must be so small that the shortcircuiting effect causedthereby will affect only a small part of the flow.

It will be obvious to those skilled in the art that many modificationsmay be made within the scope of the present invention without departingfrom the spirit thereof, and the invention includes all suchmodifications.

What is claimed is:

1. In a vertical boiler for heating non-boiling heattransfer liquidshaving two tightly wound axially symmetrically coiled pipe wallsconcentrically positioned at a distance with respect to each otherinside an axially symmetrical housing, the external wall extending tothe boiler bottom and radially delimiting the outside of a riser for theflue gas, said riser surrounding the internal wall, aperture means inthe upper end of said housing for the introduction of heating means, anda coiled pipe spiral constituting the boiler bottom,

the improvement comprising an internal pipe wall which extends to theboiler bottom, at least said internal wall with the lowermost pipewinding being supported by the boiler bottom and said lowermost windingof the internal wall extending spirally in a manner such that it restson the one hand along a spiral line against the boiler bottom and, onthe other hand, also against the next-higher helicallyextending windingof the pipe constituting the wall, and gaps between the lower coils ofsaid internal pipe wall forming a connection between the space enclosedby said internal wall and said riser.

2. A boiler according to claim 1 in which the lowermost winding of theinternal wall decreases its radius toward the wall-removed end thereof.

3. A boiler according to claim 1 including spacer means for maintainingthe gaps between the lower coils true to size.

4. A boiler according to claim 1 in which the lowermost winding of theinternal wall terminates into the boiler bottom from above.

5. A boiler according to claim 5 in which the introduction of thelowermost winding of the internal wall into the boiler bottom iseffected in a direction such that the end of a spiral channel in thebottom is rinsed by the liquid.

6. A boiler according to claim 1 in which the lowermost winding of theinternal wall terminates into the boiler bottom at the outside.

7. A boiler according to claim 1 including a third coiled pipe wall inthe riser, the coils of said wall being wound at a distance from eachother.

8. A boiler according to claim 8 including means whereby the third wallis maintained parallel to the external wall.

9. A boiler according to claim 1 including axial rods maintaining thecoiled pipe walls at a distance with respect to each other.

10. A boiler according to claim 10 in which the axial rods are connectedwith the pipe walls by frictional contact.

11. A boiler according to claim 10 in which the axial rods are connectedwith the pipe walls by formlocking contact.

12. A boiler according to claim 1 in which the internal pipe wall isconnected at the top to a return connection of the boiler and at thebottom to the external wall, which latter connects to the boilerdischarge connection at the top of said external wall.

13. A boiler according to claim 1 in which the uppermost winding of theinternal wall carries a cover of the boiler and for this purpose extendsspirally.

14. A boiler according to claim 13 in which said cover is immersed witha circumferential collar into an upwardly .open groove filled with asealing material, said groove being positioned at a sleeve surroundingthe external pipe wall.

15. A boiler according to claim 1 in which the lowermost winding of theexternal wall extends helically and is supported by means of supportingblocks welded thereto on the boiler bottom.

16. A boiler according to claim 1 in which the boiler bottom consists oftwo plates between which extends a vertical metal band spiral which isbutt-welded to one sheet whereas the other edge thereof projects througha spiral slot of the other sheet and is welded thereto from the outside.

17. A boiler according to claim 1 in which the external wall surroundsthe boiler bottom from the outside. l l

1. In a vertical boiler for heating non-boiling heat-transfer liquidshaving two tightly wound axially symmetrically coiled pipe wallsconcentrically positioned at a distance with respect to each otherinside an axially symmetrical housing, the external wall extending tothe boiler bottom and radially delimiting the outside of a riser for theflue gas, said riser surrounding the internal wall, aperture means inthe upper end of said housing for the introduction of heating means, anda coiled pipe spiral constituting the boiler bottom, the improvementcomprising an internal pipe wall which extends to the boiler bottom, atleast said internal wall with the lowermost pipe winding being supportedby the boiler bottom and said lowermost winding of the internal wallextending spirally in a manner such that it rests on the one hand alonga spiral line against the boiler bottom and, on the other hand, alsoagainst the next-higher helically-extending winding of the pipeconstituting the wall, and gaps between the lower coils of said internalpipe wall forming a connection between the space enclosed by saidinternal wall and said riser.
 2. A boiler according to claim 1 in whichthe lowermost winding of the internal wall decreases its radius towardthe wall-removed end thereof.
 3. A boiler according to claim 1 includingspacer means for maintaining the gaps between the lower coils true tosize.
 4. A boiler according to claim 1 in which the lowermost winding ofthe internal wall terminates into the boiler bottom from above.
 5. Aboiler according to claim 5 in which the introduction of the lowermostwinding of the internal wall into the boiler bottom is effected in adirection such that the end of a spiral channel in the bottom is rinsedby the liquid.
 6. A boiler according to claim 1 in which the lowermostwinding of the internal wall terminates into the boiler bottom at theoutside.
 7. A boiler according to claim 1 including a third coiled pipewall in the riser, the coils of said wall being wound at a distance fromeach other.
 8. A boiler according to claim 8 including means whereby thethird wall is maintaineD parallel to the external wall.
 9. A boileraccording to claim 1 including axial rods maintaining the coiled pipewalls at a distance with respect to each other.
 10. A boiler accordingto claim 10 in which the axial rods are connected with the pipe walls byfrictional contact.
 11. A boiler according to claim 10 in which theaxial rods are connected with the pipe walls by form-locking contact.12. A boiler according to claim 1 in which the internal pipe wall isconnected at the top to a return connection of the boiler and at thebottom to the external wall, which latter connects to the boilerdischarge connection at the top of said external wall.
 13. A boileraccording to claim 1 in which the uppermost winding of the internal wallcarries a cover of the boiler and for this purpose extends spirally. 14.A boiler according to claim 13 in which said cover is immersed with acircumferential collar into an upwardly open groove filled with asealing material, said groove being positioned at a sleeve surroundingthe external pipe wall.
 15. A boiler according to claim 1 in which thelowermost winding of the external wall extends helically and issupported by means of supporting blocks welded thereto on the boilerbottom.
 16. A boiler according to claim 1 in which the boiler bottomconsists of two plates between which extends a vertical metal bandspiral which is butt-welded to one sheet whereas the other edge thereofprojects through a spiral slot of the other sheet and is welded theretofrom the outside.
 17. A boiler according to claim 1 in which theexternal wall surrounds the boiler bottom from the outside.